Method and apparatus for removal of particles from suspension

ABSTRACT

Method and apparatus comprising an intake of bridge pier like concrete construction for water-works located in a flowing stream of water. The intake has a passage leading from a submerged inlet upstream to upper and lower downstream outlets. The passage widens in a vertical plane from the inlet towards the outlets and has a shape to cause the overall velocity of the stream to be slowed and the upper part to move faster than the lower part thereby to entrain frazil ice or other particulate material entering the passage and to carry it through the upper outlet while water free from such material passes through the lower outlet. Means is provided for injecting air near the inlet to form a curtain of air bubbles to further slow down the stream and to enhance the differential velocity between its lower and upper parts.

United States I Patent Michel et al. 45 Sept, 26, 1972 [54] METHOD ANDAPPARATUS FOR 2,728,457 12/1955 Clarke ..210/74 REMOVAL OF PARTICLESFROM 3,494,475 2/1970 Hedstrom et al. .....209/ 157 X SUSPENSION R b F dPrima Examinereu en rie man [72] Inventors: Bernard Michel, 793 DesVignes St., Granger St. Foy Quebec; J. Alllson Delaney, Att0mey LawrenceL Field 625 Milton St., Montreal, Quebec, q of Canada 57 ABSTRACT [22]Filed: Aug.2l, 1969 Method and apparatus comprising an intake of bridge21 A L N J 851 963 pier like concrete construction for water-works 1 pp0 located in a flowing stream of water. The intake has a r passageleading from a submerged inlet upstream to [52] US. Cl....'.. ..2l0/84,210/170 upper and lower downstream outlets. The passage [51] Int. Cl...B0ld 21/00 widens in a vertical plane from the inlet towards the [58]Field of Search MO/170,154, 155, 156, 83, outlets and has a shape tocause the overall velocity of 2 0 4; '15 9 74 221 259 D](; 21; thestream to 'be slowed and the upper partto move 61/2, 220, 22]; 209/ 156,[57 faster than the lower part thereby to entrain frazil ice or otherparticulate material entering the passage and 5 References Cited tocarry it through the upper outlet while waterfree from such materialpasses through the lower outlet. UNITED STATES PATENTS Means is providedfor injecting air near the inlet to v form a curtain of air bubbles tofurther slow down the 3,469,402 9/1969 Lowd ..2l0/DlG. 21 Stream and toenhance the differential velocity 3,353,679 11/1967 Hll'SCh ..2l0/259between its lower and upper parts. 2,590,756 3/1952 Colin et al...209/156 X 2,825,691 3/1958 Fischer ..210/221 X 13 Claims, 8 DrawingFigures .4 A -.4..-; A -r I 5 a;

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PNENTED I973 3.693, 796

sum 1 BF 2 INVENTORS Joseph Allison DELANEY Bernard MICHEL ATTORNEYPKTENTED 3.693.796

sum 2 or 2 MAX/MUM fill/[PAGE MIN/MUM FIG. 8 INVENTORS Joseph AllisonDELANEY Bernard MICHEL ATTORNEY METHOD AND APPARATUS FOR REMOVAL OFPARTICLES FROM SUSPENSION This invention relates to the removal ofparticulate foreign matter from flowing liquid.

When an open body of shallow water (river, stream or lake) is subjectedto freezing temperatures and the flow is relatively fast, the waterloses heat by evaporation to an extent that super-cooling occursresulting in the formation of small crystals of ice of various shapesand sizes normally termed frazil ice. Due to the high flowing velocityof the water, the frazil ice remains in a near homogeneous solutionbecause of turbulence, eddy current, etc. in the main body of water andusually enters intakes as frazil slush. Frazil slush is a concentratedorcoherent form of frazil ice. Similarly, other suspended material alsotends to enter the intake as a homogeneous suspensioninto which it isformed by turbulence. r a

v .The problem of removing frazil slush has long plagued municipalitiesand industry and has not been solved. It is an aim of. this invention toprovide means for removing frazil slush and other suspended solids atwater intakes.

At the present state of the art of intake design, for example, apipeline or tunnel is extended from shore, terminating by an intake inthe stream or having a fixed structure projecting either below or abovethe water surface. Such intakes are provided with racks and intercepttrash, including frazil slush and other particulate material. I

The present invention provides a method and apparatus that is primarilyintended to remove trash in the form of ice particles or other suspendedmaterials, automatically', by hydraulic means. The device takes the formof a solid structure, preferably of reinforced concrete, placed in thepath of a streamof water and provided with a passage leading from anentrance below water level to separate lower and upper outlets, theformer for water freed from entrained material, the latter for watercarrying material removed from the .stream. The entrance is designed toprovide for the entry of a given volume and velocity of water dependenton the desired flow. The configuration of the passage is such that ithas a floor and a roof which diverge-so as to increase thecross-sectional area and decrease the flow velocity. The floor diversionangle is such that under hydraulic operating conditions, minimumvelocity is attained, dependent on the material to be removed. Thediversion angle of the roof is related to hydraulic requirements tomaintain a continuous velocity field, such that the upper portion of theflow continues at an almost constant velocity and leaves through thedownstream outlet. The flow velocity is induced by virtue of the venturieffect of the flowing stream of water passing downstream behind and ateach side of the structure or, alternately, in the case of a dam, by theflow of the exit water caused by the pull of gravity. By virtue of thisdesign, the material particles in the stream going through the passage,because of the decreased velocity in the lower section, will rise and becarried out by the higher velocity of the flow in the upper section andwill pass out through the upper outlet. Water, freed from suspendedmaterials,

will pass to the lower outlet to the place of its intended use.

The invention will be described in more detail by reference to theaccompanying drawings which illustrate preferred embodiments and inwhich:

FIG. 1 is a plan view of a typical intake structure of a water supplysystem;

FIG. 2 is a vertical cross-section along the line 2-2 of FIG. 1; r I

FIG. 3 is a rear elevation of the intake shown on the previous figures;

FIG. 4 is a front elevation of the intake shown on the previous figures;

FIG. 5 is a vertical cross-section through another typical intakestructure for water-works;

FIG. 6 is a vertical cross-section along the line 66 of FIG. 5;

FIG. 7 is a rear elevation of the intake-shown in FIGS. 5 and 6;

FIG. 8 is a front elevation of the intake shown in FIGS. 5,6and7.

Referring more particularly to FIGS. 1 to 4, one specific form ofmunicipal water intake structure is shown as a casting A of concrete.The'casting A is partly embedded in and rests on the bed B of a movingbody of water. The casting A is provided with a passage C which leadsfrom an inlet or mouth indicated generally as D to two outlets indicatedas E and F respectively. The casting is also provided with access spaceG which can house control apparatus.

The particular shape of the mouth D and the passage C in the prototypeshown can be seen in the drawings. In longitudinal verticalcross-section, the mouth D gives somewhat the impression of a fish mouthbeing provided with a prominently projecting lower jaw 37 and spacedabove it an upper jaw 35 which does not project so far. The jaws 35 and37 are thus spaced apart both in the horizontal as well as the verticalplane and their surfaces and an extension thereof across the mouth wouldform an ice breaker nose.

The mouth D has lips 15 which extend inwardly and downwardly between thejaw 35 and the jaw 37. The jaw 35 also projects downwardly and the jaw37 upwardly to form a restricted opening at the middle of the mouth D.The shape of the passage C gradually changes in cross-section from themouth D until it becomes substantially an oval near the entrance of theoutlets E and F as shown in dotted lines in FIG. 3.

The passage C has a floor 25 which slopes gradually downwards toward theoutlet F. The passage C has a ceiling 27 which slopes upwards to outletE. The floor may be provided behind the entrance D with a sand or graveltrap 29 and close to the outlet F with a sand trap 31.

Sunk into the floor 25 are a number of spaced-apart headers 33 providedwith orifices opening into the passage C from which air can be emitted.

A splitter projection 39 or so-called splitter is provided between theoutlets E and F, functioning to split the stream between these twooutlets.

The headers 33 are pipes which extend transversely of the floor 25 andhave a surface exposed on the surface of this floor. These pipes areconnected to an apparatus, not shown, including an air saturation tankand a pump which pumps water and entrained air to the tank to provide asaturated solution of air and water. This saturated solution is pumpedinto the headers 33 and released to their orifices into the passingstream. As the air saturated water is released from the headers 33 thepressure is reduced and consequently the air is released into the streamas fine air bubbles.

OPERATION The operation of the intake shown in FIGS. 1 to 4 is asfollows. The water from the stream flows into the mouth D withentrainedfrazil slush and any other suspended materials there may be.The speed at which the water enters the passage C is dependent on howmuch is taken off at F and E respectively; In one application dealingwith frazil slush, the speed at the inlet is about 2 1% feet per second.Because of the shape of the mouth D and the passage C the velocity isreduced along the bottom surface or floor 25 to four-tenths of a footper second in this particular application relating to frazil slush. Whenthe-water hits the mouth D, the flow is faster in .the higher parts nearthe sidewalls l5 and faster between the protruding jaws 35, '37. Inother words, the construction in the middle causes the water to flowfaster than it does at the sides.

As the slope of the floor 25 starting at the lip 37 changes, thevelocity of the water adjacent to the floor 25 reduces. At the sametime, the water near the ceiling 27 tends to maintain its velocitybecause it travels directly to and through the outlet E from which it issucked by the surrounding stream of the flowing water, which can be thatof a river or discharged by the pull of gravity in the case of a darn.As the water passes along the floor over the headers 33, air is releasedfrom these headers 33 into the water which becomes entrained with thefrazil slush and suspended material, rendering the ice and material morebuoyant. This buoyancy causes the solids to rise toward the surface andto be carried out with the faster moving water leaving the outlet Edownstream. The water underneath, freed from frazil slush and othersuspended materials, flows down through the outlet'F.

Meanwhile, the traps 29 and 31, if present, trap gravel and said broughtin by the stream. The traps 29 and 3l'are evacuated by hydraulic means,as well understood. The jaws 35 and 37 divert chunks of ice flowing inthe stream.

In this particular installation, which is illustrated by way of example,the size of the mouth is roughly 50 feet wide by 12 feet high and thelength of the passage between the front of the mouth to the splitter 39is about 100 feet (the drawing being out of proportion).

The special shape contour of the passage accomplishes good separationfrom the entering stream of frazil slush and other suspended material,without the injection of air. Air injection is an added optionalfeature.

An alternate form of intake is shown in FIGS. 5 through 8. To facilitatereference to the various parts they have been numbered with tens anddigits the same as in FIGS. 1 to 4 but raised by one hundred and withthe parts shown by letters, the letters are the same but have been shownwith a prime sign.

In this form of the invention, the upper lip 135 protrudes beyond thelower lip 137. In the form of device shown in FIGS. 1 to 4, the waterlever is always well above the bottom of the lip 135 and the top of thislip 135 slopes upwards for the purpose of diverting ice over its top aswell as to each side. In the construction shown in FIGS. 5 andfollowing, the nose of the inlet D' formed by the lips and 137 isdesigned rather to divert ice to either side of the structure. The lowerlip 137 has a vertical front face to prevent rocks from jumping the lipinto the passage 1 15.

The cross-section of the passage from inlet D towards the outlets E, Fis nearly rectangular, as shown in FIG. 7, with the side walls verticaland a fillet between the side walls and the roof 137 and floor 125.

The angle of inclination of the floor 125 is not more than about 7degrees to the horizontal near the inlet but increases sharply aroundthe mid-point of the floor 125 and the angle of inclination of theceiling from the horizontal is from about 1 degree to about 5 degrees. Asplitter 139 would also be provided between the outlets E and F in themanner as illustrated in FIG. 5 to split the stream between these twooutlets E and F The outlet E is preferably divided into two separatepassages 140 and 141 formed in the end wall 144 and having the contourshown.

In the particular form shown in FIGS. 5 to 8, the sand traps shown in.FIGS. 1 to 4 have been eliminated. Heavy solids like sand and gravelwill pass down along the floor into the outlet F where they can becollected by a conventional trap in the bottom of a vertical pit, whilethe feed water is drawn off through a horizontal tunnel having itsentrance above the bottom of the pit.

In this form of the invention there are spaced-apart headers 133provided with air openings. The headers 133 run from side to side alongthe floor 125, as shown, leading to the passage 115 and adapted to jetair into the water from the floor 125. The headers 133 are fed from asuitable source of air under pressure. The headers 133 have spaced-apartopenings, from which air, from a suitable source of compressed air, is

' released, to form curtains of bubbles.

The water intake shown is essentially an ice flotation device withoutmechanical parts. The elimination of the ice starts even before thewater enters the intake at D. The upper part of the entrance opening issituated 10 feet lower than the lowest water level, so that the floatingicicles, breaking drift ice, a good part of the snow slush and compactflakes of frazil are brushed to one side on'the surfaces in front of theentrance. Also, there is considerable slowing of the speed of flowbefore the entrance where the speeds change from 6 or 7 feet per secondto 2 or 3 feet per second at the entrance. This accentuates the processof external removal of the compact flakes of frazil which are located ata lower level.

In the interior of the passage C the flow diverges progressively over alength of approximately feet. The speeds are reduced by more than halfand the flakes of frazil float towards the ceiling of the intake.Further, the active frazil which has been able to gain entrance, hastime to develop stable crystals which are no longer adhesive when theyreach the opening of the vertical intake channel.

The two openings 140 and 141 act as hydraulic aspirators, 10 feet by 10feet, in this example, to permit the maintaining of higher speeds at thetop of the interior passageway so as to aspirate and evacuate the frazilat the downstream end of the intake.

During this time the part of the flow used would be 350 million gallowsper day, split towards the horizontal entry of the outlet F The speedsare in principle reduced to less than 1 foot a second and the flakes offrazil cannot sink into the outlet F.

The curtain of air injected into the channelway near the entry of theintake through the headers 133 is made up of bubbles which vary indiameter from micron size and upwards and have the following effects.They can adhere to and float relatively big submerged material, buttheir principal role is to create a curtain of bubbles which slows thespeed of the lower part of the flowing water, and accelerates the upperpart, to facilitate still more the flotation of the frazil.

Heat may be used as a supplementary factor in order to obtain afurthersafeguard against ice accumulation. The natural heat of the earthmay be harnessed by locating the entrance of the tunnel about 100 feetbelow the bed of the river At this depth the temperature of the rock isin the neighborhood of 44 F during the winter.

However, to take care of the extremely improbable eventuality of asizable deposit of ice on the interior walls of the intake, radiantheating may be installed on the walls at the most exposed places, nearthe entrance.

The lower lip of the entrance at the front nose of the intake iselevated 4 feet above the bed of the river so as to prevent large rocksfrom entering into the intake. If, by chance, gravel and sand or both isentrained in the intake, it will deposit at the bottom of the verticalshaft leading from F. The entrance of the water supply tunnel issituated above the bottom of this shaft and leads laterally from itswall. A sediment pump may be used as required for periodically removinggravel and sand deposits from the bottom of the shaft.

EXAMPLE The following is an example of a typical inlet constructionaccording to the invention. The inlet structure is substantially asshown in FIGS. 5 to 8 similar to a bridge pier and its passage has alength of, from entrance D to outlet E, I50 feet, a width between theside walls 115 or 50 feet and a height of around 12 feet at the inletand is made of reinforced concrete. The construction was accomplishedsimilarly to constructing a bridge pier by building a coffer dam in astream to form an enclosure. The water was removed and the bed excavatedto provide a receiving cavity in which forms were placed and concretepoured in the usual manner to provide the structure. Once the concretehad set the forms were stripped.

The coffer dam was then removed. The stream then both flowed around thestructure and entered the inlet, as shown in the drawings. One billiongallons a day flowed into the inlet. Entrained in the water entering theinlet at winter temperatures was a maximum of about 4 percent by volumeof frazil slush. Air was injected through two headers spaced apart asshown at 133 in FIGS. 5 to 8 each extending the whole width of the floorand having openings about one-sixteenth of an inch in diameter spaced 1foot apart. There were 45 orifices of both headers. There was a totaldischarge from the orifices of both headers of 34 cubic feet per minuteof air.

The amount of water leaving the outlet F was approximately 350 milliongallons per 24 hour day. The amount leaving the outlet E wasapproximately 650 million gallons per 24 hour day. The velocity of thewater was 3 feet per second near the entrance D and 1 foot per second onthe floor as it approached the outlet F and about 6 feet per second nearthe outlet E. No substantial amount of frazil slush was entrained in thestream passing through the outlet F. (End of example.)

The specific installations shown and described are by way of example andare adapted to specific conditions. It will be understood, of course,that the exact construction for a given installation may vary within thespirit of the invention, depending on requirements in terms of volume ofwater, depth of water, velocity, conditions in the stream, and theamount of ice and other debris to be removed.

While the structure has been shown as forming a part of a water-workswater intake, the same principle may be applied to a continuousstructure, for example a dam intake to turbines. In a dam, the outlet Ewould lead to the overflow at the crest of the dam, and, the passage Fwould lead to a turbine. The suction applied to the water at E wouldthen result from the force of gravity acting on the water falling overthe crest of the dam. The amount of water lost, flowing through E, is arelatively small portion of the amount going through F. The devices ofthe invention can operate with varying water levels and varying volumesand rates of flow.

We claim:

1. An apparatus to be positioned in a body of flowing liquid forseparating finely divided solids entrained in the flowing liquid, saidapparatus comprising a structure having a passage defined by an upperwall, a lower wall and a pair of spaced-apart side walls, an inlet insaid structure leading downstream into said passage, said inlet being atleast below the surface of a body of liquid from which finely dividedsolids are to be separated, said upper wall having portions narrowingfrom said inlet to said outlet, said lower wall having portions wideningfrom said inlet to said outlet, said side walls having portions wideningin their vertical plane from said inlet to said outlet, said structurehav ing upper and lower vertically spaced-apart outlets in communicationwith said passage downstream of said inlet, said upper outlet beingadjacent said upper wall and communicating with the body of flowingliquid, and said outlet being adjacent said lower wall whereby adifferential flow velocity is established between the liquid dischargingfrom said upper and lower outlets for discharging the entrained solidsthrough said upper outlet and discharging generally solid free liquidthrough said lower outlet, and splitter means in said passage betweensaid outlets for splitting the differentially flowing liquid between theupper and lower outlets.

2. The apparatus of claim 1 further characterized in that the upper wallportion diverges upwardly at an angle such that the upper strata flowcontinues at an almost constant velocity.

3. The apparatus of claim 1 further characterized in that the lower wallportion diverges downwardly at an angle such that underhydraulicoperating conditions a minimum flow velocity is achieved.

4. The apparatus of claim 1 further characterized in that thespaced-apart side walls are substantially nondivergent.

5. The apparatus of claim 1 further characterized in that the finelydivided solids are frazil ice and the stream of liquid is water.

6. The apparatus of claim 1 in which means is provided for injecting airfrom the lower wall near the inlet to form a curtain of bubbles risingthrough the liquid in the passage adapted to assist flotation of thefinely divided solids.

7. The apparatus of claim 1 in which the downward slope of the lowerwall is greater in zones downstream of the mid point of the passage thanzones upstream thereof.

8. The apparatus of claim 1 in which the splitting means comprises atapered baffle projecting upstream to orient the upper and lower stratabetween the respective upper and lower outlets;

9. An installation for freeing a body of flowing liquid of finelydivided solids entrained in the flowing liquid, comprising, an apparatuslocated in the bed of a body of flowing liquid, said apparatuscomprising a structure having a passage defined by an upper wall, alower wall and a pair of spaced-apart side walls, an inlet in saidstructure leading downstream into said passage, said inlet being atleast below the surface of a body of liquid from which finely dividedsolids are to be separated, said upper wall having portions narrowingfrom said inlet to said outlet, said lower wall having portions wideningfrom said inlet to said outlet, said side walls having portions wideningin their vertical plane from said inlet to said outlet, said structurehaving upper and lower vertically spaced-apart outlets in communicationwith said passage downstream of said inlet, said upper outlet beingadjacent said upper wall and communicating with the body of flowingliquid, and said outlet being adjacent said lower wall whereby adifferential flow velocity is established between the liquid dischargingfrom said upper and lower outlets for discharging the entrained solidsthrough said upper outlet and discharging generally solid free liquidthrough said lower outlet, and splitter means in said passage betweensaid outlets for splitting the differentially flowing liquid between theupper and lower outlets.

10. The installation of claim 9 wherein the body of flowing liquid is anatural body of water and the finely divided solids generally compriseice particles.

11. A method of substantially separating finely divided solids entrainedin a body of flowing liquid comprising the steps of: reducing theoverall velocity of at least a portion of said liquid by introduction ofsaid portion of said liquid into a submerged inlet of a structure havinga passage defined by an upper wall, a lower wall and a pair ofspaced-apart side walls, and a plurality of downstream outlets,establishing a differential velocity between upper and lower strata ofsaid portion of said liquid due to said upper wall having portionsnarrowing from said inlet to said outlets, said lower wall havingportions widening from said inlet to said outlets, said side wallshaving portions widening in their vertical plane from said inlet to saidoutlet, said side walls having portions widening in their vertical planefrom said inlet to said outlets wherein the velocity of the upper strataexceeds the velocit of the lower strata whereby entrained solids aresubs antially concentrated n the upper strata; discharging saidconcentrated entrained solids containing strata back into said body offlowing liquid; and passing the lower substantially entrained solidsfree strata to a point of use.

12. The method of claim 11 including the step of injecting a gaseousphase-containing fluid into said portion of said liquid during said stepof reducing the overall velocity of said portion of said liquid to forma curtain of bubbles rising through the liquid to assist the flotationof the finely divided entrained solids into the upper stratum.

13. The method of claim 11 wherein said flowing liquid is water and saidentrained solids generally comprise ice particles.

2. The apparatus of claim 1 further characterized in that the upper wallportion diverges upwardly at an angle such that the upper strata flowcontinues at an almost constant velocity.
 3. The apparatus of claim 1further characterized in that the lower wall portion diverges downwardlyat an angle such that under hydraulic operating conditions a minimumflow velocity is achieved.
 4. The apparatus of claim 1 furthercharacterized in that the spaced-apart side walls are substantiallynon-divergent.
 5. The apparatus of claim 1 further characterized in thatthe finely divided solids are frazil ice and the stream of liquid iswater.
 6. The apparatus of claim 1 in which means is provided forinjecting air from the lower wall near the inlet to form a curtain ofbubbles rising through the liquid in the passage adapted to assistflotation of the finely divided solids.
 7. The apparatus of claim 1 inwhich the downward slope of the lower wall is greater in zonesdownstream of the mid point of the passage than zones upstream thereof.8. The apparatus of claim 1 in which the splitting means comprises atapered baffle projecting upstream to orient the upper and lower stratabetween the respective upper and lower outlets.
 9. An installation forfreeing a body of flowing liquid of finely divided solids entrained inthe flowing liquid, comprising, an apparatus located in the bed of abody of flowing liquid, said apparatus comprising a structure having apassage defined by an upper wall, a lower wall and a pair ofspaced-apart side walls, an inlet in said structure leading downstreaminto said passage, said inlet being at least below the surface of a bodyof liquid from which finely divided solids are to be separated, saidupper wall having portions narrowing from said inlet to said outlet,said lower wall having portions widening from said inlet to said outlet,said side walls having portions widening in their vertical plane fromsaid inlet to said outlet, said structure having upper and lowervertically spaced-apart outlets in communication with said passagedownstream of said inlet, said upper outlet being adjacent said upperwall and communicating with the body of flowing liquid, and said outletbeing adjacent said lower wall whereby a differential flow velocity isestablished between the liquid discharging from said upper and loweroutlets for discharging the entrained solids through said upper outletand discharging generally solid free liquid through said lower outlet,and splitter means in said passage between said outlets for splittingthe differentially flowing liquid between the upper and lower outlets.10. The installation of claim 9 wherein the body of flowing liquid is anatural body of water and the finely divided solids generally compriseice particleS.
 11. A method of substantially separating finely dividedsolids entrained in a body of flowing liquid comprising the steps of:reducing the overall velocity of at least a portion of said liquid byintroduction of said portion of said liquid into a submerged inlet of astructure having a passage defined by an upper wall, a lower wall and apair of spaced-apart side walls, and a plurality of downstream outlets,establishing a differential velocity between upper and lower strata ofsaid portion of said liquid due to said upper wall having portionsnarrowing from said inlet to said outlets, said lower wall havingportions widening from said inlet to said outlets, said side wallshaving portions widening in their vertical plane from said inlet to saidoutlet, said side walls having portions widening in their vertical planefrom said inlet to said outlets wherein the velocity of the upper strataexceeds the velocity of the lower strata whereby entrained solids aresubstantially concentrated in the upper strata; discharging saidconcentrated entrained solids containing strata back into said body offlowing liquid; and passing the lower substantially entrained solidsfree strata to a point of use.
 12. The method of claim 11 including thestep of injecting a gaseous phase-containing fluid into said portion ofsaid liquid during said step of reducing the overall velocity of saidportion of said liquid to form a curtain of bubbles rising through theliquid to assist the flotation of the finely divided entrained solidsinto the upper stratum.
 13. The method of claim 11 wherein said flowingliquid is water and said entrained solids generally comprise iceparticles.